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BUREAU OF MINES 
INFORMATION CIRCULAR/1988 



Application of Stereoscopic (3-D) 
Slides to Roof and Rib Hazard 
Recognition Training 



By Edward A. Barrett, William J. Wiehagen, 
and Robert H. Peters 



UNITED STATE? DEPARTMENT OF THE INTERIOR 



Information Circular 9210 



Application of Stereoscopic (3-D) 
Slides to Roof and Rib Hazard 
Recognition Training 



By Edward A. Barrett, William J. Wiehagen, 
and Robert H. Peters 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
T S Ary, Director 







't 



Library of Congress Cataloging in Publication Data: 



Barrett, Edward A. 

Application of stereoscopic (3-D) slides to roof and rib hazard recognition 
training. 

(Bureau of Mines information circular; 9210) 

Bibliography: p. 12. 

Supt. of Docs, no.: I 28.27:9210. 

1. Mine safety— Study and teaching— Audio-visual aids. 2. Ground control (Min- 
ing). 3. Mine roof control. 4. Transparancies in education. 5. Photography, Stereo- 
scopic. I. Wiehagen, William J. II. Peters, Robert H. III. Title. IV. Series: Infor- 
mation circular (United States. Bureau of Mines); 9210. 



TN295.U4 



622 s [622\8] 



88-600263 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Acknowledgments 3 

Ground hazard recognition training 3 

Feasibility of using 3-D slides as a training aid 4 

Effectiveness of 3-D slides 5 

3-D photography 6 

Original equipment 6 

Stereoscopic camera slide bar 7 

Assembling and viewing 3-D slides 8 

Cost and availability of 3-D equipment 8 

Equipment recommendations 8 

Recommendations for training using 3-D slides 11 

Summary 11 

References 12 

Appendix-Design drawings for stereoscopic camera slide bar 13 

ILLUSTRATIONS 

1. 3-D cameras 6 

2. 3-D slide projector and 3-D glasses 7 

3. Handheld 3-D slide bar 7 

4. Slide bar for taking 3-D slides with standard 35-mm camera 7 

5. Slide projectors vertically positioned on a two-tier shelf 9 

6. Polarizing filter and filter holder 10 

7. Aluminum 3-D slide holder and cardboard mount 10 

A-l. Stereoscopic camera slide bar assembly 13 

A-2. Stereoscopic camera slide bar detail 14 

A-3. Stereoscopic camera slide bar flash bracket detail 15 

TABLES 

1. Effectiveness of 3-D slides in portraying groundfall hazards 4 

2. Approximate costs and sources of 3-D equipment 8 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


ft 


foot 


mm 


millimeter 


in 


inch 


pet 


percent 



APPLICATION OF STEREOSCOPIC (3-D) 

SLIDES TO ROOF AND RIB HAZARD 

RECOGNITION TRAINING 

By Edward A. Barrett, 1 William J. Wiehagen, 2 
and Robert H. Peters 3 



ABSTRACT 

The ability to recognize visual cues commonly associated with roof and rib hazards is fundamental 
to the prevention of groundfall accidents. The perceptual skills that miners possess to visually search 
and assess hazardous ground conditions differ considerably. This U.S. Bureau of Mines report summa- 
rizes recent investigations on the use of stereoscopic (3-D) slides as a training aid for improving the 
ability of miners to recognize the various geologic and mining-induced irregularities that cause ground- 
falls. The feasibility of using 3-D slides for training and their effectiveness for representing roof and rib 
hazard conditions are discussed. Information is presented on equipment and procedures for creating 
a 3-D slide program that can be updated by mine trainers. Recommendations are made on selection 
of 3-D equipment and on the use of 3-D slides for training miners to recognize roof and rib hazards. 



Mining engineer. 
Supervisory industrial engineer. 
3 Research psychologist. 
Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



Many factors influence the likelihood of groundfall 
accidents. Some, such as geological factors that establish 
the inherent stability of the roof and rib and hidden irreg- 
ularities that become exposed as mining advances, he be- 
yond the control of the underground worker. However, 
miners can maintain some level of command over these 
factors and their effect on groundfall accidents by having 
the ability to recognize and correct potential hazards. 
Recognition of hazards means that the worker perceives 
the warning associated with the problem, recognizes the 
warning for what it actually represents, and assesses the 
risk involved. Correction of hazards includes actions such 
as avoiding the problem area, seeking assistance in ad- 
dressing the problem, or actually making physical changes. 
Each of these actions constitutes an attempt on the part of 
the miner to deal with, or correct, the problem. It is es- 
sential for their individual safety that all workers be capa- 
ble of recognizing roof and rib hazards. Clearly, ground 
hazard conditions affect everyone who works underground, 
regardless of specific job responsibilities. The potential for 
danger exists everywhere; therefore, all miners have a need 
to be competent in the detection of roof and rib hazards 
as they perform tasks underground or simply commute to 
and from work stations. 

To illustrate, in a review of recent Mine Safety and 
Health Administration (MSHA) accident and fatality re- 
ports, it was noted that groundfall accidents may have been 
prevented in some cases had workers been able to detect 
the presence of hazardous features and properly assess the 
risk. For example, consider the following conclusions filed 
by MSHA inspectors following two separate investigations: 
"the accident occurred when a piece of undetected roof 
(horseback formation) fell from between the roof bolts 
causing fatal injuries to the victim" and "the contributing 
factor to the accident and resultant fatality was the pres- 
ence of an undetected kettlebottom near the face of No. 5 
entry." Of course, it is not possible to ascertain the visual 
capabilities of the persons involved in these incidents with 
any reasonable degree of accuracy. The visual information 
available to the miners at the moment of the accident may 
have been occluded or perhaps the persons did indeed 
recognize the hazards but failed to assess the degree of 
danger or simply ignored the hazard. The point is that 
opportunities exist to improve the perceptual abilities of 
underground workers and to increase their capacity for 
making sound judgments about how to deal with perceived 
dangers. 

Further evidence of the need for better training in the 
identification of groundfall hazards can be found in two 
recent Bureau studies. The first study investigated the 
human factors contributing to groundfall accidents in un- 
derground coal mines (7). 4 This report summarized the 
views of miners, section supervisors, and mine inspectors 

Italic numbers in parentheses refer to items in the list of references 
at the end of this report. 



regarding what they think should be done to reduce the 
frequency of injuries sustained in falls of roof and rib. In 
one portion of the study, 143 miners from nine different 
underground coal mines were asked to indicate the degree 
to which each of nine possible strategies would help them 
avoid rockfall injuries. The following six-point rating scale 
was used: 1— very small, 2-small, 3-somewhat small, 
4-somewhat large, 5-large, 6-very large. The list included 
strategies such as adding more support to bad areas of 
roof, better installation of roof bolts, better training in 
proper methods of supporting the roof, and better training 
in the identification of roof hazards. Sixty-eight percent of 
the miners said, to a large or very large degree, that better 
training in identifying roof hazards would help them avoid 
rockfall injuries. 

The miners were also asked several questions about 
their experiences with rockfall accidents. Sixty-two percent 
(88 out of 143) reported having had some type of recent 
experience with a rockfall. These included rockfalls that 
resulted in injuries and those that were close enough to 
startle nearby workers. These data suggest that unplanned 
rockfalls in underground coal mines are all too frequent 
events. In such environments, miners need to understand 
and be familiar with potentially hazardous roof and rib 
conditions. Moreover, of the 88 incidents reported, 66 
occurred within approximately 25 ft of the face and 52 
happened within a few minutes after the worker had ar- 
rived at the area. This suggests that many rockfall ac- 
cidents may be avoided if workers would examine the roof, 
particularly before initiating work in a new area. 

The implication here is that in order for hazardous 
ground conditions to be corrected, the miner must first be 
capable of identifying the problem and correctly assessing 
the potential risk. The inability of the worker to visually 
search, identify, and judge the degree of danger of obvious 
as well as subtle roof hazards can significantly increase the 
odds of unplanned, unanticipated roof failures and subse- 
quent accidents. 

In the second study, an empirical test was designed to 
measure the effectiveness of 3-D slides for depicting 
ground hazards (2). One of the research objectives was to 
determine if underground experience is related to the vis- 
ual skill level of miners. Results from this portion of the 
study indicated that in viewing a set of 3-D slides of 
groundfall hazards a group of experienced miners failed to 
correctly identify the hazard in approximately 17 pet of the 
slides. Another group of persons with limited under- 
ground working experience failed to correctly identify the 
hazard in approximately 33 pet of the same set of slides. 
While ability does improve with experience, these results 
indicate that miners, even experienced ones, sometimes fail 
to recognize certain types of potentially dangerous roof 
and rib conditions. These findings imply a need for im- 
proved ability in the perception and recognition of ground- 
fall hazards. 



ACKNOWLEDGMENTS 



The authors thank Henry J. Kellner, industrial engineer- 
ing technician, Pittsburgh Research Center, and Marion B. 
Molchen, design draftsman, Boeing Services International 



Inc., for their assistance and cooperation in 3-D equipment 
design and numerous field evaluation exercises. 



GROUND HAZARD RECOGNITION TRAINING 



All miners are required by law to receive training in a 
prescribed array of mine health and safety topics. The 
ground control portion mandates, as a minimum, a review 
of the roof or ground control plans in effect at the mine 
and, for some, instruction on procedures for dealing with 
ground control problems. At some mines, the training 
programs include roof and rib hazard recognition or 
awareness training. Examples of the former may include 
how to bar down loose roof with a pry bar or how to 
sound roof. The latter may involve, in addition to the 
recognition of hazards such as geologic irregularities and 
loose rock occurrences, learning particular skills and gain- 
ing knowledge for determining the proper course of action 
to pursue and when to take the action. 

To determine an appropriate course of action, discrim- 
ination of relevant cues is required as well as recall and 
identification of applicable rules and concepts. The per- 
ceptual abilities and skills of the miner are invoked in 
order to observe these cues. A comprehensive ground 
control training program would ideally include a combi- 
nation of these elements; that is, information that is re- 
quired by law (roof control plan and ground control pro- 
cedures) and skills training in the recognition of roof and 
rib hazards. The following scenario, taken from a recent 
MSHA fatality report, illustrates the need for such a com- 
prehensive ground hazard recognition training program. 

On April 2, 1985, a miner helper was killed by a fall of 
roof while installing temporary support for setting line 
brattice inby permanent support. The accident involved 
a 7-ft by 7-ft by 7-in-thick slab of the immediate roof shale. 
Investigators reported that although the fallen slab was not 
a true kettlebottom, it was similar in shape. It was nearly 
circular in plan view and was bounded on one edge by a 
slickcnsided surface. One side of the slab coincided with 
a portion of a flattened, carbonized fossil tree trunk that 
was oriented parallel to the shale bedding and was approx- 
imately 20 in wide. It appeared that the rock separated 
along the outby (slickcnsided) edge, along the plant fossil, 
and along a shale lamina. The rock then sheared along 
the inby edge near the rib. The investigators concluded 
that the victim, who had 36 years of mining experience, 
failed to detect the loose rock condition and placed himself 
in an unsafe position while advancing the line brattice. 

The question naturally arises as to whether the victim 
could have been trained to a level of skill that would have 
prevented making the error that led to his death. Theoret- 
ically, the answer is yes. However, it is unlikely that simply 
giving him more of the same kind of training would 



have sufficed. What could be more effective is the oppor- 
tunity to practice perceptual and judgmental skills in a 
training environment designed to simulate, to a high de- 
gree of fidelity, ground conditions that a miner needs to 
learn to recognize. 

Traditional visual aids used in mine training classes- 
films, overhead projections, videos, and slides-are two- 
dimensional (2-D) models that represent the three-dimen- 
sional world. Some trainees, particularly new miners, are 
unable to form mental images of hazardous ground condi- 
tions so that transfer of learning can occur when the per- 
son goes underground. 

The need for classroom simulations of real conditions 
has long been recognized in the training industry. Gibson, 
for instance, suggested that in industrial training there 
should be devices that would simulate particular dangers 
while allowing subjects to act safely or unsafely (3). One 
problem with simulations, however, is that they often have 
an artificiality that is difficult to surmount. This is partic- 
ularly true with roof and rib hazards. Moreover, as these 
hazards are represented in 2-D images, they become less 
authentic looking and quite unlike the real entity. Con- 
founding this simulation problem is the complex mine en- 
vironment where the visible cues are subtle, constantly 
changing, and often masked (rock dust, bad viewing angle, 
limited lighting). Nevertheless, classroom training effec- 
tiveness and transfer must depend on instructional mate- 
rials that physically simulate the real world environment. 
This is generally true for all types of training and, in par- 
ticular, for mine roof and rib hazard training because of 
the nature of inherent hazards and extremely variable 
conditions. 

One attempt to simulate ground hazards in mining was 
carried out in 1979 by Blignaut who asked subjects to per- 
form motor tasks while simultaneously looking for loose 
rock in a stope simulator (4). Although Blignaut reported 
that the simulator was viewed as realistic by the partici- 
pants, such a device would be relatively difficult and quite 
expensive to build with any degree of fidelity. Blignaut 
followed this with another attempt to simulate ground haz- 
ards that appears to offer more promise to mine safety 
trainers. This second attempt involved the use of 3-D 
slides to train underground miners. The results of his 
study involving South African gold miners suggest that the 
ability of underground miners to discriminate between 
dangerous and safe rock conditions can be improved sig- 
nificantly by exposing them to 3-D slides of groundfall 
hazards. 



Although Blignaut's results were encouraging, they 
failed to provide guidelines for using 3-D slides as training 
aids to teach hazard recognition skills. Therefore, the 
Bureau undertook a pilot study to determine the efficacy 
of using 3-D slides for coal mine ground hazard recogni- 
tion training. 

The first step was to produce a representative set of 
3-D slides of hazardous roof and rib conditions typically 
found in coal mines. A variety of hazardous underground 
conditions were photographed in 3-D at mines throughout 



the major coal producing areas of the eastern United 
States. The slides contained geologic features, such as 
joints, bedding planes, and kettlebottoms; inadequate sup- 
port conditions, such as spalling ribs, loose or hanging 
bolts, and incorrect bolting patterns; and loose rock occur- 
rences, such as overhangs. The second step was a research 
study to investigate the feasibility of utilizing 3-D slides as 
a tr ainin g aid for improving the miner's ability to recognize 
hazardous roof and rib conditions. 



FEASIBILITY OF USING 3-D SLIDES AS A TRAINING AID 



Four feasibility issues were investigated: Acceptance of 
3-D slides by trainers and trainees, compatibility with cur- 
rent ground control instruction methods, effectiveness of 
3-D slides as a training aid, and availability, cost, and 
reliability of 3-D photographic and projection equipment 
(5). Data were collected using a structured interview 
guide and two sets of slides, one in 2-D and the other in 
3-D, of common, easily recognized ground hazards. The 
data were obtained from mine managers, company and 
union officials, Federal and State inspectors, mine safety 
and training personnel, and miners. The information gen- 
erated in the first three areas of the investigation was 
essentially a collection of observations from the respon- 
dents based on intuition after viewing the slides in a hand- 
held viewer. This provided data for establishing the face 
validity of 3-D slides for training. Complete details of this 
feasibility study are reported in reference 5. A discussion 
of the segment of the study relating to the effectiveness of 
3-D slides from the perspective of mine safety and training 
personnel follows. 

In the study, data were collected on the plausible use 
of 3-D slides as a training aid for ground control from 47 
persons who attended MSHA's 1985 National Mine In- 
structor's Conference, from 9 MSHA ground control spe- 
cialists, and from 143 miners working underground. Vir- 
tually all of these people agreed that 3-D slides were much 
better than 2-D slides for portraying critical features of 
hazardous areas of roof and rib, and that 3-D slides would 
make an excellent training aid. A portion of the response 
information offered by those who participated in the con- 
ference is outlined here. The opinions of this group are 
meaningful because (1) group members were or had been 
directly associated with miners' training and, hence, should 
be good judges of the effectiveness of 3-D slides as a train- 
ing aid and (2) they represented the hierarchy of persons 
in a mining company who could decide if 3-D slides should 
be added to their training programs. 

The conference attendees were asked a series of ques- 
tions about the effectiveness of 3-D photography and the 
overall feasibility of using 3-D slides as a mine training 
aid. Each participant was asked to view eight slides of 
hazardous roof or rib conditions. Each slide was pre- 
sented first in the conventional 2-D form, and then in 3-D 
form. 



The first question asked was: "To what extent are 3-D 
slides more or less effective than conventional (2-D) slides 
at portraying groundfall hazards?" Responses to this ques- 
tion (table 1) indicated that most participants (93.3 pet) 
believe that 3-D slides are a great deal more effective than 
2-D slides at portraying groundfall hazards, and that a very 
small percentage (2.2 pet) think they are less effective. 

Participants were also asked: "What benefits (if any) 
would there be to adding 3-D slides illustrating groundfall 
hazards to your company's safety training programs?" The 
benefits that were most often listed include 

Would provide a more realistic representation of 
hazards. 

Would be more interesting to the trainees. 

Would make it easier to teach miners about hazards. 

Would increase miners' abilities to recognize hazards. 

Would help to generate more discussion. 

Participants were then given several statements concern- 
ing how most miners would react to the addition of 3-D 
slides to their mine safety training and asked to circle each 
statement with which they agreed. Of the sample, 68 pet 
believed that 3-D slides would improve miners' abilities to 
detect hazardous conditions; 66 pet thought that miners 
would enjoy seeing a new and unique type of training slide; 
57 pet expected that 3-D slides would elicit more discus- 
sion about the hazards portrayed; and 46 pet thought 
miners would view 3-D slides as an attempt to upgrade 
their training. Only one person indicated that 3-D slides 
would have no effect on miner's performance. 

TABLE 1. - Effectiveness of 3-D slides in portraying 
groundfall hazards 

Category label pet 

Much more effective than 2-D slides 71.1 

More effective than 2-D slides 22.2 

Slightly more effective than 2-D slides 4.5 

The same as 2-D slides 

Less effective than 2-D slides 2.2 



Finally, participants were asked to suggest other mine 
training applications for 3-D photography in addition to 
illustrating groundfall hazards. Specific safety and task 
tr ainin g applications noted were new miner orientation, 
preblasting and postblasting surveys, and accident 
investigations. 

The three most important advantages of 3-D slides over 
2-D slides for teaching ground hazard recognition skills 
indicated by the workshop participants were (1) they illus- 
trate groundfall hazards more realistically, (2) they make 
it easier to teach this area of training, and (3) they can 



make ground control training, in general, more interesting 
for miners. Overall, the feasibility study concluded that it 
is indeed possible for mine trainers to use 3-D slides of 
hazardous mine roof and rib conditions as a training aid 
for teaching the worker to recognize groundfall hazards. 
Specific concerns noted in this study focused on the ques- 
tion of availability and practicality of existing 3-D photo- 
graphic equipment. These were addressed in followup 
Bureau investigations and are presented in a subsequent 
section of this report. 



EFFECTIVENESS OF 3-D SLIDES 



There are few references on the utilization of 3-D slides 
for instructional purposes in the literature, and those spe- 
cifically reporting on the application of 3-D slides for 
ground hazard recognition training were limited to the 
Blignaut study previously reported. To supplement Blig- 
naut's work, a pilot study was conducted to determine the 
effectiveness of 3-D slides for the recognition of ground 
hazards. 

A field experiment was designed with the primary ob- 
jective of determining if visual cues associated with ground 
control hazards are more apparent when represented in 
3-D slides than in 2-D slides (2). A secondary objective 
was to determine if underground experience has some ef- 
fect on the visual skill levels of miners. The experimental 
strategy adopted for the investigation resulted in a 2 by 2 
factorial design (diagrammed below) in which mode of 
stimulus presentation (3-D or 2-D) was experimentally 
combined with the level of experience (experienced or 
inexperienced) of the miner. 



Experienced miners 
Inexperienced miners 



The two independent variables, subsequently referred to 
as the main effects in the experiment, were the type of 
stimulus material (3-D or 2-D) and the level of experience 
of the observer (experienced or inexperienced miner). 
Forty subjects participated in the experiment, 10 in each 
cell. Twenty miners had underground experience ranging 
from 2 years to 39 years. The 20 inexperienced subjects 
were persons with casual and limited underground experi- 
ence, that is, either students in a mining engineering cur- 
riculum or researchers on mining projects. None of the 
latter had worked underground; however, all were familiar 
with mining to some degree. Each subject was asked to 
view a set of 15 slides. The slides were prejudged by 
experts to be those of relatively common ground hazards. 
Members of half of each of the two groups (either experi- 
enced or inexperienced) were shown the roof and rib haz- 
ards using 3-D slides, and the remaining group members 
were shown identical 2-D slides. After viewing the slides, 



3-D 


2-D 











each subject was asked to respond as follows: (a) describe 
the hazard, (b) indicate the degree of danger of the hazard 
(lethal, high, mild, or minimum), and (c) indicate how the 
hazard can be corrected. Correct responses to each query 
are shown in the following individual cells. 

Describe the Hazard 



Experienced 
Inexperienced 



Indicate the Degree of Danger of the Hazard 



3-D 


2-D 


125 


96 


101 


66 



3-D 


2-D 


86 


64 


93 


66 



Experienced 
Inexperienced 



Indicate How the Hazard Can Be Corrected 



3-D 


2-D 


124 


73 


97 


64 



Experienced 
Inexperienced 



The correct responses given by all 40 subjects to each 
query were treated using a 2 by 2 analysis of variance 
(ANOVA) to determine the significance of both main 
effects-level of experience and mode of stimulus pre- 
sentation. Complete summary tables for the two-way 
ANOVA can be found in reference 2. From these data it 
was concluded that both main effects were statistically sig- 
nificant (p <0.01). In comparison to the group of persons 
with limited underground experience, the proportion of 
correct responses was significantly higher among experi- 
enced miners. In comparison to the group who viewed 
the slides in 2-D, the proportion of correct responses was 
significantly higher among those who viewed the slides in 
3-D, both for experienced and for inexperienced miners. 

The first of these two findings suggests that significant 
differences exist between the ability of new versus exper- 
ienced miners to correctly identify groundfall hazards. 



An important footnote to this finding is that, on the 
average, even the experienced miners failed to correctly 
identify 2.5 out of 15 hazards. This suggests that better 
training in recognizing groundfall hazards could benefit 
experienced miners as well as miners who have little or no 
underground experience. 



The second finding clearly denotes that 3-D slides are 
more effective for the purposes of illustrating groundfall 
hazards. Therefore, it can be concluded from this study 
that 3-D slides are a better medium than 2-D slides for 
illustrating groundfall hazards for all miners. 



3-D PHOTOGRAPHY 



ORIGINAL EQUIPMENT 

Stereo photography dates back to the mid- 19th century 
with the invention of the stereoscope (stereo viewer) in 
England by Sir Charles Wheatstone. However, until the 
introduction of Kodachrome film in 1936, stereo photog- 
raphy had limited appeal and use, other than hobby, due 
mainly to poor technical quality and inferior equipment. 
Interest was revived in 1947 with production of the Realist 
stereo camera; Kodak, Wollensak, Delta, Coronet, TDC, 
and other manufacturers followed with similar 3-D camera 
designs (fig. 1). 

Stereoscopic cameras are 35-mm slide cameras that 
have dual, matched objective lenses with mechanically cou- 
pled iris diaphragms. They have a normal range of aper- 
ature settings, shutter speeds, shoe adapters, and focus 
adjustments. Any standard color or black and white slide 
film in the common range of ASA speeds may be used. It 
should be noted here that these are slide cameras and 
cannot be used for taking 3-D photographs. A four-lens 
camera system is required for taking 3-D prints. 

Stereoscopic slides can be viewed either on a screen or 
through handheld devices. The equipment required to 
project and view 3-D slides on a screen include a 3-D slide 
projector and polarized 3-D glasses (fig. 2). For this, a 
special viewing screen must be used that has a lenticular 
surface. The screen image from a 3-D slide projector is 
not as clear as that observed in a handheld 3-D viewer 
(fig. 3). Polarized glasses must be worn to see projected 
3-D slides; however, polarized glasses are not required 
with the handheld viewer. 



Stereo slide film is developed in the same manner as 
other 35-mm film; however, the preparation and mounting 
of 3-D slides is done by relatively few film processing 
companies because the work is time-consuming and costly. 
This, of course, presents a problem, not only with origi- 
nals, but when duplicate sets of slides are required for 
training purposes. 

Several concerns exist regarding the acquisition and 
functional capabilities of 3-D cameras and projectors. Per- 
haps the most critical problem with 3-D cameras is limited 
availability; they are not currently being manufactured, as 
commercial production was terminated more than 30 years 
ago. Reconditioned cameras may, however, be found in 
some used equipment departments of photographic supply 
stores. Occasionally, advertisements can be found in pho- 
tography magazines to purchase 3-D cameras through mail 
order. Some 3-D cameras have also become collector's 
items among stereo enthusiastics. Manufacture of 3-D 
slide projectors also was discontinued many years ago; 
however, they too can occasionally be found in used equip- 
ment departments of camera shops. The available supply 
of used 3-D projectors is considerably less than that of 
3-D cameras because fewer companies manufactured them 
originally. 

In addition to the problem of supply, 3-D cameras have 
several operational disadvantages when compared with 
modern 35-mm slide cameras. These include built-in 
lenses that are not interchangeable, antiquated manual 
controls, and difficult focusing adjustments (particularly in 
low light settings). The first two shortcomings can be 
overcome by using the slide bar discussed in the next 





Figure 1.— 3-D cameras. 





Figure 2.— 3-D slide projector and 3-D glasses. 




Figure 3.— Handheld 3-D slide viewer. 



section. Setting the correct focus for a subject in the dim 
underground environment can be accomplished by using 
the distance, f-stop linear calibrations indicated on the 
strobe light. 

STEREOSCOPIC CAMERA 
SLIDE BAR 

In order to surmount these 3-D camera problems and 
also to simplify the procedures for generating 3-D slides, 
the Bureau designed and fabricated a slide bar for produc- 
ing 3-D slides using available photographic equipment. 
The stereoscopic camera slide bar (6) uses just one 35- 
mm, single lens reflex camera to take matched pairs of 
individual slides (left slide and right slide) from two preset 




Figure 4.— Slide bar for taking 3-D slides with standard 35-mm 
camera. 



locations (fig. 4). These positions correspond to the inter- 
ocular distance between lenses, 2.75 in, used on the orig- 
inal, dual lens 3-D cameras. The unit basically consists of 
three horizontal bars situated one above the other. The 
lower bar is used for mounting the unit on a tripod and for 
holding a simple line level. The middle bar holds the 
camera, which is attached to a track-mounted sliding plate, 
and the upper bar supports the strobe light that is secured 
at a location central to each of the two camera positions. 
Detailed design drawings for construction of the slide bar 
are included in the appendix. The bar can be constructed 
of aluminum or steel stock and contains no parts that are 
unusually difficult to cut. The complete device can be 
fabricated and assembled in-house and requires no partic- 
ular skills other than those typically found among workers 
in a mine repair shop. Description of the slide bar oper- 
ation follows. 

The bar, attached to a standard tripod for stability, is 
balanced horizontally using the line level. The left slide is 
then taken after the camera is moved to the leftmost posi- 
tion on the middle bar. A strobe light, mounted on the 
top bar and connected to the camera via a shutter cord, is 



fired from its fixed location. After "sliding" the camera to 
the rightmost position on the bar, the right slide is taken. 
The strobe light is again fired from the same fixed posi- 
tion, insuring that shadows in each slide are identical and 
balanced. After taking a pair of slides, a blank slide is 
included in the sequence before taking the next set; this 
eliminates confusion during the slide assembly process. 

An important feature available when using the stereo- 
scopic camera slide bar is the opportunity to change cam- 
era lenses to suit the occasion. Original 3-D cameras have 
built-in, fixed lenses that are not removable. Thus, by 
using either a zoom lens, a telephoto lens, or a wide angle 
lens, the subject in the 3-D slide can be captured from a 
range of distances and directions without moving the tri- 
pod and camera from its original position. This is an 
efficient method for generating multiple slides of a partic- 
ular hazard, where each slide can represent a different 
field condition such as variable lighting, multiple shadows, 
projection from roof or rib, etc. Safety is also a consider- 
ation here. It is much safer for the photographer to film 
a closeup view of a hazardous roof condition using a zoom 
lens from a secure distance of perhaps 30 ft than assuming 
a position near to the danger area. 

ASSEMBLING AND VIEWING 
3-D SLIDES 

The matched 35-mm slide pairs can either be projected 
on a screen or mounted and observed in handheld viewers. 
To project the slide pairs on a screen (lenticular type 
only), two conventional slide projectors can be positioned 
vertically and the lenses angled slightly so the images over- 
lap on the screen (fig. 5). The lenses used in each projec- 
tor should be similar and have polarizing filters (No. 7945 
are acceptable) on each lens (fig. 6). If handheld viewers 
are used, then the individual 35-mm film chips must be 
mounted in aluminum stereo slide holders and cardboard 
stereo mounts (fig. 7). Both of these items are available 
in photographic supply stores. The mounting process, 
which follows, is quite basic and can be accomplished by 
a novice. 

The film chips are placed in the aluminum holder so 
the left chip has slightly more subject exposed on the left 
edge of the slide; conversely, the right chip should show 
slightly more subject on the right edge. The holders are 
manufactured so that a fair amount of sideways adjustment 
is possible. In a trial and error process, the slides are 
alternately adjusted and viewed until the proper stereo 
balance is achieved. It will be obvious to the observer 
when the proper stereo position of the chips has been 



reached because the image will show depth and be clear 
throughout. 

COST AND AVAILABILITY 
OF 3-D EQUIPMENT 

The list of equipment in table 2 includes items that may 
be used to set up a 3-D slide training program, in-house. 
The costs are presented merely as a guideline; obviously 
they will vary, depending on quality, quantity, sizes, and 
sources. Standard items such as a 35-mm camera, conven- 
tional slide projectors, strobe light, and shutter cord are 
not noted. 

TABLE 2. - Approximate costs and sources 
of 3-D equipment 

Item Cost Source 

3-D camera $250 Camera stores; mail order. 

3-D projector 550 Do. 

Lenticular screen 110 Camera stores. 

3-D glasses 5 Do. 

Handheld viewer 20 Do. 

3-D slide bar NAp Notcommerciallyavailable. 

Stereo slide holders and 7 Camera stores. 

mounts. 1 
Polarizing filter 13 Do 

NAp Not applicable, ^ox of 50. 

EQUIPMENT RECOMMENDATIONS 

The most practical approach for training departments 
to set up a 3-D slide program is to acquire the items 
needed for producing 3-D slides using the slide bar. This 
represents the most advantageous direction because most 
materials necessary for startup are available either in- 
house or from camera shops, and the slide bar can be 
made in a local machine or mine repair shop. 

A complete inventory would include the following items: 
slide bar, 35-mm camera, stereo mounts, handheld viewers, 
35-mm slide projectors, lenticular screen, and 3-D glasses. 
The last three items can be excluded if handheld viewers 
are chosen at the mode of presentation. This approach is 
suggested if the training classes are small, say fewer than 
10 or 15 persons. For larger classes, projecting 3-D slides 
on a screen may be the most efficient mode of presenta- 
tion. In either case, original 3-D cameras are not recom- 
mended for producing the set of slides. The quality of 
stereo slides using the slide bar is far superior to those 
taken with a 3-D camera, primarily because of the lens in- 
terchange feature. For this reason, as well as for that of 
equipment availability, the slide bar offers greater flexibil- 
ity for developing, in-house, a 3-D slide training program. 







Figure 5.— Slide projectors vertically positioned on a two-tier shelf. 



10 




Figure 6— Polarizing filter and filter holder. 





Figure 7.-Aluminum 3-D slide holder and cardboard mount. 



11 



RECOMMENDATIONS FOR TRAINING USING 3-D SLIDES 



Because of the inherent advantage of 3-D slides, that is, 
to represent with impressive vividness the length, width, 
and depth of objects, they are a potentially valuable train- 
ing aid for teaching hazard recognition skills in the class- 
room. Stereo slides offer a high-fidelity medium for rep- 
resenting real mine conditions. 

There are several instructional advantages for advocat- 
ing 3-D slides as a training aid. One major advantage for 
using 3-D slides in training is they ostensibly take the 
trainee into the mine without ever leaving the classroom. 
This is particularly important for ground control training. 
Providing instruction in the recognition of roof and rib 
hazards in an on-the-job mode is not always practical, 
efficient, or complete. Moreover, teaching hazard recog- 
nition on-the-job carries certain risks, particularly when 
the penalty for error may be extremely high. 

A dilemma here is that many hazards may never get 
taught simply because they do not exist underground at the 
time of training, or, if they do exist, their nature or stage 
of development may vary from that under study. For ex- 
ample, cutter roof will initially appear as short separations 
in the mine roof running in the direction of the opening 
along the rib line. Eventually, they will develop into much 
longer separations that run on both sides of the entry for 
extended distances. Finally, an entire area affected by 
cutter roof will fail in shear and collapse at, or above, the 
roof bolt anchorage horizon level, unless appropriate sup- 
port is installed. It is unlikely that all stages of this hazard 
would be represented in any mine at one given time. By 
using 3-D slides to depict each of several development 
phases of cutter roof, the trainee can be taught to recog- 
nize and assess this hazardous condition and subsequently 
make informed decisions on corrective actions to take at 
an appropriate time. 

Another instructional advantage for using 3-D slides is 
that because of the availability of photographic equipment 
and the simple procedures for making slides, a 3-D ground 
control training program can be assembled and continually 



updated to meet training objectives by in-house personnel. 
This is particularly attractive to mine trainers because they 
can generate customized sets of 3-D slides that depict 
ground hazard conditions found in their own mine, per- 
haps even on the current working section, and do so with 
a minimum investment of company resources. Moreover, 
these customized slides create additional interest among 
trainees as they view familiar surroundings and, conse- 
quently, become more actively involved in training class 
discussions and more motivated to learn. 

Stereoscopic slides can also play an important role in 
assessing or quantifying the visual skill levels of miners 
prior to training. By determining the capability of each 
worker in recognizing ground hazards initially, training can 
thus become more meaningful and efficient. In addition, 
learning outcomes of training and subsequent retention can 
be easily measured and evaluated. The ability of miners 
to perceive roof and rib hazards can be ascertained by 
using a representative set of 3-D slides that depict ground 
hazards found in their mine. Training that follows could 
be selective, that is, it would address those deficiencies 
diagnosed during the pretraining exercise. This, in effect, 
would establish a knowledge base for each person and a 
point of departure for subsequent training efforts. 

A final comment on the use of 3-D slides for training 
relates to an often overlooked segment of most training 
programs, that of reinforcement of demonstrated safe con- 
ditions. Positive reinforcement can serve as an effective 
mechanism for enhancing any subject lesson. Stereo slides 
may be used to accomplish this objective by displaying 
3-D scenes that contain no hazards; in other words scenes 
showing proper support, adequate scaling, uniform pillar 
corners with minimum sloughing, etc. These would serve 
to reinforce acceptable roof and rib conditions having 
proper support techniques. The high fidelity of 3-D slides 
in approximating the underground environment makes 
them quite suitable for this purpose. 



SUMMARY 



Miners possess varying degrees of hazard recognition 
skills that have been acquired through job experiences and 
training. Evidence exists that miners sometimes fail to 
recognize areas of hazardous roof and rib. Stereoscopic 
slides offer a unique medium for teaching and evaluating 
these skills. They realistically portray the natural mine 
environment and provide excellent proxies for miners 
learning to recognize the visual cues that characterize 
unstable ground conditions. 

There are several advantages for using 3-D slides in- 
stead of conventional visual aids for illustrating groundfall 
hazards to miners. Perhaps the most important one is that 



3-D slides provide a more accurate representation of roof 
and rib hazards and, because of their realistic appearance, 
they are intrinsically interesting to trainees. Miners seem 
to take a great deal of interest in training that involves 3-D 
slides and seem to enjoy looking at this type of slide. 
Based on the attitudes of miners and mine trainers, it 
appears that 3-D slides would be very well received as a 
training aid to complement the regular training program. 
The equipment needed to produce 3-D slides is rela- 
tively inexpensive, easy to obtain, and basic to operate. 
Any standard 35-mm, single lens reflex camera can be 
used for taking ordered pairs of slides. The procedures for 



12 



mounting these pairs of film chips in 3-D slides holders 
can be accomplished by a novice with a minimum amount 
of practice. Most of the materials required for the com- 
plete 3-D process are available at photographic supply 
stores. 

Some of the more important characteristics affecting the 
acceptance of any innovative mining technology by the in- 
dustry are simplicity, availability, and cost. To this end, 
the Bureau has advanced and adapted the state-of-the-art 



of 3-D photographic equipment and procedures for docu- 
menting roof and rib hazards to the level that many mine 
training departments can inexpensively provide and main- 
tain their own materials. Information presented in this 
report suggests that it is both feasible and advisable for 
trainers to use 3-D slides of hazardous roof and rib as a 
training aid for improving the miner's ability to recognize 
potentially dangerous ground conditions. 



REFERENCES 



1. Peters, R H., and W. J. Wiehagen. Human Factors Contrib- 
uting to Groundfall Accidents in Underground Coal Mines: Workers' 
Views. BuMines IC 9127, 1987, 24 pp. 

2. Barrett, E. Behavioral Aspects of Roof/Rib Injuries— Impli- 
cations for Training Utilizing Stereoscopic Photography. Paper in Pro- 
ceedings of Fifth Conference on Ground Control in Mining (WV Univ., 
Morgantown, WV, June 11-13, 1986), ed. by A. W. Khair and S. S. 
Peng. WV Univ., 1986, pp. 213-220. 

3. Gibson, E. J. Principles of Perceptual Learning and 
Development. Prentice-Hall, New York, 1969, 515 pp. 



4. Blignaut, C. The Perception of Hazard: The Contribution of 
Signal Detection to Hazard Perception. Ergonomics, v. 22, 1979, 
pp. 1177-1183. 

5. Peters, R The Feasibility of Using Stereoscopic Photography 
To Improve Miners' Training on Groundfall Hazards. Paper in Pro- 
ceedings of Fourth Annual Meeting of the Collegiate Association for 
Mining Education (Univ. Missouri, Rolla, MO, Oct. 3-4, 1985). CAME 
1986, pp. 2-17. 

6. Barrett, E. A., M. B. Molchen, and H. J. Kellner. Stereoscopic 
Camera Slide Bar. U.S. Pat. 4,768,049, Aug. 30, 1988. 



13 








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